Ballast is the granular material usually broken stone or brick, shingle or kankar, gravel or sand. It provides a suitable foundation for the sleepers. It also holds the sleepers in their correct level and position. The lateral stability of a track depends on it.

Functions of ballast

Ballast performs the following functions –

• Transfers the load from the sleepers to the subgrade and then distributes it uniformly over a larger area of the formation.
• Holds the sleepers in position and prevents the lateral and longitudinal movement due to load transfer.
• Imparts some degree of elasticity to the track.
• Provides easy means of maintaining the correct levels of two lines of a track.
• Corrects track alignment.
• Provides good drained foundation immediately below the sleepers and help to protect the top surface of the formation.

Requirements / Characteristics of the good ballast

To perform above mentioned functions, the ballast should have following characteristics –

• Able to withstand hand picking without disintegrating.
• Resist crushing under dynamic loads.
• Therefore, it should not be able to make the track dusty or muddy.
• Must have good drainage with minimum soakage.
• Voids should be large enough to prevent capillary action.
• Offer resistance to abrasion and weathering.
• Retain its position laterally and longitudinally under conditions of traffic, specially on curves.
• Should not produce any chemical action with rail and metal sleepers.
• Size of stone should be 5 cm for wooden sleepers, 4 cm for metal sleepers, 2.5 cm for turn outs and cross-overs.
• Easily available so that it reduces the cost of supply.

Types of Ballast

Different materials used as ballast in India are broken stone, gravel, sand, ashes, cinders. And sometimes soft aggregates like moorum, kankar, overburnt and broken brickbats, blast furnace slag and selected earth.

1. Broken Stone

This is the best material for ballast and almost all important tracks are provided with stone ballast. It satisfies all the specifications and requirements of good ballast as mentioned above. The best stone is nonporous, hard and angular, which does not flake when broken. Igneous rocks such as hard trap, quartzite, granite make excellent ballast. These are used in large quantities for high speed tracks on India.

2. Gravel / Shingle / River Pebbles

Gravel comes next in rank for its suitability for use a ballast; thus used in large quantities in many countries. This is obtained either from river beds or from gravel pits. The smooth pebbles are broken, otherwise they are liable to displace the sleeper due to smoothness.

The process of ramming the ballasts underneath the sleeper is known as Packing. The ballasts above this layer which surrounds the sleeper, is loose filled and termed as Boxing. The loose ballasts between the two adjacent sleepers is Ballast Crib.

3. Ashes / Cinders

This material is available in large quantities on railways from coal being used in locomotives. It has excellent drainage properties as it is very porous. It is cheap and largely used in sidings. But can not be used for main lines as it is very soft and reduce the powder on track. It is excellent for station yards and for footpaths. It doesn’t retain water and is not slippery.

Greater drawback is, it’s corrosive in nature. Therefore it corrodes steel sleepers and foot of the rails. In emergencies, due to its easy availablity, can be used for repairing formation or packing tracks.

4. Sand

Its reasonably good material as it is cheap and provides good drainage. Sand also produce a silent track and is good for packing pot sleepers.

Greater drawback of the sand is its blowing effect due to vibration. Sand easily gets on track, thus causes heavy wear. Therefore the maintenance of track is difficult. Sometimes sand layer is covered with stone or brick, to prevent it from blowing. Mostly coarse sand is preferred to fine sand.

5. Moorum

It is the soft aggregate and is the result of decomposition of laterite. It is red or sometimes yellow in colour. The best moorum is that which contains large quantities of small laterite stone.

It is recommended for sidings and main track. Specially when they are new laid and the embankments are not sufficiently consolidated. When moorum is finally laid on the track, it forms a soling or blanket under the stone.

6. Kankar

It is lime agglomerate which is common in certain clayey soils and is dug out of the ground. Where stone is not easily available, it is used as road metal and as ballast for railway tracks. It is soft in nature and reduces to powder under loads. It is used for M.G. and N.G. tracks with light traffic and where a better type of the ballast is not available.

7. Brick Ballast

Where no stone or substitute is available for use as ballast, overbunt bricks are broken into small sizes and used. It powders easily and produces a dusty track. Rails of tracks laid on brick ballasts many a time get corrugated. Brick ballasts, however, is fairly good for drainage.

8. Blast furnace slag

Which is a by-product in the manufacture of pig iron forms a suitable ballasts material. It should, however, be hard, of high density and free from gas holes. Slag, suitable for use as ballasts, is obtained by pouring molten slag collected at the blast furnace into shallow pits of thin layers, allowing it to cool, and then by digging, crushing and screening.

9. Selected Earth

For sidings, earth, if of suitable quality, is sometimes used as ballasts. It is also sometimes used on new formation as a temporary measure. Indurated (i.e. hardened) clay and decomposed rock are suitable materials.

Size and Section of ballast

The size of the ballasts used varies from 1.9 cm to 5.1 cm gauge. Stones of larger sizes are not desirable and the maximum size as 5.1 cm is preferable, as interlocking of stones of this size is better than that of stone of larger sizes. The best ballast is that which contains stones varying in size from 1.9 cm to 5.l cm with reasonable proportion of intermediate sizes.

The exact size of the ballast depends upon the type of sleeper used and location of the track as below –

• Ballast size for wooden sleepers tracks = 5.1 cm. (gauge)
• Ballast size for steel sleepers tracks = 3.8 cm. (gauge)
• Ballast size for under switches and crossings = 2.54 cm. (gauge)

The section of ballast layer consists of depth of ballasts under the sleepers and the width of the ballasts layer.

The depth of the it under the sleepers is an important factor in the load bearing capacity and uniformity of distribution of load. The more is the depth of ballast more will be the load-bearing capacity. In America, a depth of it equivalent to the sleeper spacing is recommended, because of heavier loads and the closer spacing of sleepers being used in that country. In India, this recommendation will give unnecessarily thicker layer of ballast due to large spacing of sleepers, being used.

The width of the ballast layer is also important as the lateral strength of track depends partly upon the quantity of ballasts used at the ends of the sleepers. The lateral strength increases with increase in width of the ballast layer but there is a limit beyond which no useful purpose is served by widening. This width limit is at 38 cm. to 43 cm. from the end of the sleeper as computed. The Indian standards have recommended dimensions for (i) width of ballast section at the level of the foot of the rail and (ii) for depth of ballast below the sleepers.

Minimum Depth of Ballast Section

Although the lines of equal pressure in ballast through wheel loads are in the shape of a ‘bulb’ yet for simplicity purpose, the load dispersion can be assumed at 45° to the vertical. For uniform distribution of load on the formation, the depth of ballast should be such that the dispersion lines do not overlap each other. From simple geometry of the following figure, the depth of ballast can be calculated as below –

Sleeper spacing (S) = width (w) + 2 x Depth of ballast (Db)
or S = w+ 2 x Db
Minimum Depth of ballast = Db = (S-w)/2

For example, if wooden sleepers are used in track laying with sleeper density as (n +7), the sleeper spacing is 65 cm and width of sleeper is 25 cm.

Then, the minimum depth of ballast from above formula works out to be 20 cm which is minimum depth of ballast generally prescribed on Indian Railways.

Quantity of ballast

The quantity of stone ballasts required per metre tangent length is 1.036 m for B.G., 0.71 m for M.G. and 0.53 m for N.G. of track with standard ballast cushion and also timber sleepers. However, slight variations in quantities occur with the type of sleeper used. Wooden and concrete through sleepers because of their bulk, require slightly less ballasts than pot or trough sleepers.

For curves with super-elevation, the quantity of ballasts is slightly more, because as per the Indian standards, recommended depth at level is provided under the inner edge of the sleeper. Moreover, wider shoulders (about 15 cm or more) than specified above are provided on outside curves to counteract the increased lateral thrust.

Renewal of Ballast

Due to prolonged and repeated movement of wheel loads over the railway track, the ballasts material gets crushed to smaller sizes. The fine crushed particles get filled in the voids and form an impervious layer, preventing the rapid flow (i.e. drainage) of the rain water. It thus creates drainage problem.

Quantity of ballast is also reduced due to –

(a) Blowing away of the ballast by the movement of trains.
(b) Penetration of the ballast in the formation, and
(c) Loss of ballast due to rain water and wind etc.

To make up the loss, the ballast is renewed from time to time. In the process of renewal, the crushed fine material is removed by screening, through the screens as shown in following figure and new material of the desired size is added.